Science Briefs

Mineral Clues to Past Climates

By Vivien Gornitz —
November 2005

Earth's climate has shifted dramatically and frequently during the last few million years, alternating between ice ages, when vast glaciers covered northern Europe and much of North America, and interglacials — warm periods similar to the present. New York City was buried under several thousand feet of ice as recently as 10,000 years ago. Relics of the last ice age are still visible in Central Park: glacial erratics (large boulders dragged and dumped by glaciers from upstate New York), glacial grooves or striations gouged into rocky outcrops, and roches moutonnées (Fr. sheep-like rocks) smoothly abraded on the side from which the glacier came and roughly plucked on the lee side. Terminal moraines — rocky, sandy ridges of debris left by the retreating glaciers — criss-cross parts of Staten Island and Long Island. Farther back in time, climates have differed even more from the present.

Geoscientists use "proxies", or indirect means to reconstruct climates that existed long before the invention of thermometers, barometers, or other meteorological instruments: tree rings, pollen grains, animal and plant fossil assemblages. Changes in oxygen isotope ratios (18O/16O) in the calcitic shells of tiny marine organisms like foraminifera can be linked to fluctuations in ocean temperatures and sea levels. Levels of greenhouse gases (carbon dioxide, methane) trapped in layers of ice from Greenland and Antarctica rise and fall in sync with interglacial-glacial cycles.

Minerals also furnish important clues about ancient climates. At Earth's surface, minerals interact closely with water and the atmosphere. Most useful are those deposited under relatively narrow climatic ranges or within specific environmental settings. These include evaporites, low temperature minerals such as ikaite and hydrohalite, minerals of residual soils (e.g., in bauxites or laterites), and some clay minerals like kaolinite.

Signs of Aridity

Gypsum, var. selenite (Excalibur Mineral Corportation specimen).

Evaporite minerals form by evaporation of seawater or lakes in narrow basins, rift valleys (like the East African Rift Valley), and coastal lagoons under extremely hot and dry climates. Their distribution closely matches that of deserts. As water evaporates from the basin, salts precipitate in a sequence usually starting with carbonates, sulfates, and ending with the more soluble chloride salts. Typical evaporite minerals include gypsum, anhydrite, halite (rock salt), borax, and nitrates, such as saltpeter or niter (potassium nitrate). Major deposits of rock salt occur in the Gulf Coast, the Austrian Alps, the Dead Sea, upstate New York, Michigan, and elsewhere.

Gypsum and anhydrite are among the most abundant evaporite minerals. Gypsum also crystallizes within muds in desert playas and coastal tidal pools in arid regions. When buried under several hundred meters of sediments, it dehydrates to anhydrite. Anhydrite can also precipitate directly from highly saturated brines at temperatures above 22°C. Mirabilite is a very soluble evaporite mineral that precipitates under highly saline conditions, and also at low temperatures. Halite crystallizes in supersaturated brines.

Other signs of aridity are vast accumulations of chemically-resistant minerals like quartz that survive weathering in nearly all but very hot and humid climates. In deserts (e.g., the Sahara), winds pile quartz grains into dunes and huge sand seas (ergs). Fossil desert sands include the Jurassic Navajo and Kayenta Sandstones of Utah and Colorado and the Permian Coconino Sandstone of the Grand Canyon region, northern Arizona. The directions and intensities of former winds can be determined from the orientation of wind ripples, cross-bedding, and grain sizes preserved in the fossil sand dunes.

Climate Clues from Soils and Sediments

Clay minerals form by the chemical break-down of rocks near Earth's surface. The detritus is removed by water erosion and accumulates in lakes, estuaries, and the sea. Clays also occur in terrestrial soils and airborne dust. The types of clay minerals and their relative abundances are closely related to climate, although the composition of the source rocks can also influence their development. Kaolinite, for example, is created by intense chemical weathering in warm, humid climates where silica is leached out, leaving soils enriched in alumina. Chlorite and illite, on the other hand, tend to form in soils dominated by mechanical weathering, both in colder, often formerly glaciated regions, but also in hot, dry climates.

Since most clays are brought to the oceans by rivers, their distribution in the oceans closely matches that of adjacent landmasses, showing a latitudinal banding corresponding to the major climate zones. Kaolinite is abundant in equatorial waters, whereas chlorite and illite concentrations increase toward higher latitudes.

Clay minerals can also record important past climate events. For example, around 33.5 million years ago, smectite and kaolinite became much more abundant relative to illite in marine sediments off the coast of Antarctica. This shift has been correlated with an abrupt change in the marine oxygen isotope record, marking the onset of widespread glaciation on Antarctica.

Bauxite (aluminum ore) from Arkansas.

Bauxite is a residual soil that forms by intense chemical weathering of rocks in wet, tropical climates where the average rainfall is 60 inches/year. The extreme leaching destroys most silicates and even resistant minerals such as quartz, leaving insoluble aluminum minerals, such as gibbsite, and boehmite, with lesser quantities of diaspore, kaolinite, and iron oxides.

Ancient Atmospheres

Elements like iron that can exist in several oxidation states may shed light on ancient atmospheres. Pyrite oxidizes and alters rapidly in today's oxygen-rich atmosphere and thus does not generally form as a result of erosion. Therefore, well-rounded grains of pyrite with uraninite, and siderite in three-billion-year-old sediments from the Witwatersrand basin in South Africa appear to indicate much lower atmospheric oxygen levels during the Archean eon (>2.5 billion years ago) than at present. However this interpretation has been controversial, since the grains could have been deposited by heated solutions much later, rather than weathering out of rock, like the gold nuggets they were found with. Yet several recent studies strengthen the case for the placer (erosional) theory. Precise dating using 187Re/187Os isotopes shows that the rounded gold grains (with associated pyrite and uraninite) are about 3 billion years old as compared to the significantly younger ages of 2.76-2.89 billion years for the enclosing host conglomerates. The well-rounded grains also show signs of mechanical abrasion. This suggests that the pyrite and uraninite formed by erosion under an oxygen-deficient atmosphere 3 billion years ago.

Cold Climates

Glendonite, calcite after ikaite, Russia (Element51 specimen).

Ikaite is a rare mineral that occurs in nature at temperatures up to 7°C in alkaline, phosphate-rich marine and continental waters. Although ikaite has generally been replaced by calcite (this replacement is called glendonite), it has preserved its original crystal shape. Because of the rather restricted conditions under which ikaite/glendonite forms, it serves as a marker for near-freezing water temperatures. Hydrohalite, a form of salt, precipitates at low temperatures from highly saturated brines. It is not well-preserved in sediments because of its high solubility, but its former existence can be inferred from the presence of halite or other minerals that preserve the shape of the original crystals.

Minerals are just one of many proxies used by geologists to reconstruct past climates. Some minerals like ikaite define a very narrow set of conditions, while others, like the clays, appear in a number of geologic and climatic settings. Minerals along with other indicators — fossils, tree rings, pollen, ice cores, geochemistry or isotopes — provide tantalizing clues to ancient climates.